Guide rods of this general type are known, for example, from German Patent publication DE 3 615, 349 A1, which deals with a gathered curtain, which has guide rods that are disposed at intervals perpendicularly to the gathering direction. Gathering cords run through the guide rods and the ends of these cords are fastened to the lowest guide rod. By operating the gathering cord, the curtain is gathered or let down. The function of the guide rod is to achieve uniform folding of the curtain during the gathering. Because of the connection between the rods and the curtain, the latter can easily be taken down from the window for cleaning purposes. The gathering cord and the guide rods remain on the window and need not be unthreaded and, later on, threaded again for this purpose. Such window decorations are however suitable only for rectangular windows and vertical window areas. For so-called horizontal roof windows, the guide rods must have lateral guide elements, which run in rails and prevent sagging of the curtain. If the window is substantially inclined with respect to the vertical, gravity would no longer suffice to overcome the friction of these guide elements when the curtain is let down. Furthermore, it would be desirable to be able to decorate also other geometric window shapes, such as triangles, circles, semicircles and domes, because such shapes occur increasingly in modern house construction, particularly in so-called winter gardens.